Neuromuscular choristomas (NMC), also known as benign triton tumors, nerve rhabdomyomas and neuromuscular or ectomesenchymal hamartomas are benign expansile peripheral nerve sheath tumors featuring a tight interconnection of skeletal muscle and nerve fibers within the endoneurial sheath. They also seem to have a precursor role in desmoid-type fibromatosis.
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Epidemiology
Neuromuscular choristomas are extremely rare and are usually seen in childhood or adolescence. There is no gender predilection 1.
Associations
Neuromuscular choristoma is associated with desmoid-type fibromatosis 1-3.
Diagnosis
The diagnosis of neuromuscular choristoma is established by histological, immunohistochemical and molecular pathological criteria respectively 1.
Diagnostic criteria
Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5th edition) 1:
- presence of skeletal muscle between nerve fascicles or intraneural components
- intact surrounding perineurium
- immunohistochemical proof of myogenic, neural and endoneurial components
- expression of ß-catenin in neuromuscular choristoma or neuromuscular choristoma
Clinical presentation
Typical clinical features include signs of neuropathy or plexopathy with muscular atrophy as well as progressive pain 1,2.
Pathology
Neuromuscular choristomas are peripheral nerve tumors where skeletal muscle fibers are positioned within a nerve and its fibers 1-3.
Etiology
Hypotheses about the etiology of neuromuscular choristomas include the following 1,3:
- hamartomatous overgrowth of muscle spindles
- intraneural entrapment of skeletal muscle during embryogenesis
- myogenic metaplasia of nerve sheath components
- intraneural rhabdomyoma
Location
Neuromuscular choristomas have been reported in large nerves or within the plexus as the sciatic nerve and the brachial plexus. They have been also described in cranial nerves or intraorbital 1-5.
Macroscopic appearance
Macroscopically neuromuscular choristoma shows a multifascicular fusiform nerve enlargement with some nerve fascicles showing a red muscle-like appearance. Nerve stimulation causes contraction of the whole nerve in addition to the innervated muscle 1.
Microscopic appearance
Microscopically neuromuscular choristoma is characterized by disorganized fascicles of skeletal muscle intertwined with nerve fascicles or neural elements 1-4.
Immunophenotype
Immunohistochemistry stains are positive for desmin and muscle-specific actin as well as S100, and possibly epithelial membrane antigen (EMA) or β-catenin 1-3.
Genetics
Most cases of neuromuscular choristomas show a CTNNB1 mutation 1,2.
Radiographic features
MRI
Typical magnetic resonance imaging features include fusiform enlargement of the nerve with characteristics resembling skeletal musculature, as long as there is no associated desmoid tumor 1-4,6.
Signal characteristics
- T1: isointense or slightly hypointense to muscle
- T2: isointense or slightly hypointense to muscle
-
T1 C+ (Gd):
- no or minimal enhancement in neuromuscular choristoma alone 1
- strong enhancement in cases with superimposed desmoid fibromatosis 1,6
Radiology report
The radiological report should include a description of the following:
- form, location and size
- tumor margins and transition zone
- affected nerve/plexus and relations to adjacent neurovascular structures
- interfascicular fat
- associated desmoid tumor if present
- relations to bones
Treatment and prognosis
Neuromuscular choristoma is benign and therefore does not require an overly aggressive approach. Since a trauma or surgery-related stimulation of desmoid-type fibromatosis has been considered and the latter requires aggressive management a do not touch approach has been advocated in patients in whom the diagnosis is clinically and radiologically clear 1,4.
History and etymology
A case of a neuromuscular choristoma of the sciatic nerve was first described by the Italian physician E. Orlandi in 1895 3,4,7.
Differential diagnosis
Conditions that may mimic the presentation and/or appearance of a neuromuscular choristoma include 1,6:
- lipofibromatous hamartoma: interfascicular fat
- rhabdomyoma
- embryonal rhabdomyosarcoma
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